High-Impact Thunderstorms of the Brisbane Metropolitan Area

• Published in: Journal of Southern Hemisphere earth systems science 2019-01, p.1
• Main Authors: Soderholm, Joshua S, Turner, Kathryn I, Brook, Jordan P, Wedd, Tony, Callaghan, Jeffery
• Format: Article
• Language: English
• Subjects: Airports; Climatology; Convection; Convective heating; Convective storms; Heating; Insured losses; Mesoscale processes; Metropolitan areas; Nowcasting; Radar; Severe weather; Storm forecasting; Thunderstorms; Warm seasons; Winds
• ISSN: 2206-5865
• DOI: 10.22499/3.6901.018

Accurate thunderstorm warnings in the hours to minutes preceding impact are often limited by the complex evolution of the mesoscale atmospheric environment. To accurately capture these complexities, analysis of observations remains central to operational short term nowcasting predictions of thunderstorms. Over the past 40 years, multiple high-impact thunderstorm events have impacted the Brisbane Metropolitan Area (BMA) of South East Queensland (SEQ) resulting significant insured losses. Four of these high-impact events are the focus of the following work. These cases include three events that resulted in the greatest insured losses for the BMA, exceeding $4 billion AUD (2017) (18 January 1985, 16 November 2008 and 27 November 2014) and a fourth significant event (24 December 1989). Synthesis of previous work indicates that the four high-impact cases occurred during a southeasterly change with strengthening winds ahead of the change, suggesting commonalities may exist that can be exploited for forecasting. This paper provides a detailed observational analysis of the environment and convective storms from the four BMA events to explore discriminating characteristics that may improve the skill of nowcasting. For the four BMA events, significant deep convection was observed along the change for the hours prior to the change's arrival at the Brisbane Airport, potentially acting as an early indicator of favourable conditions for high-impact thunderstorms. It was found that the timing of the southeasterly change through Brisbane was also highly correlated for all events, occurring within a 90 minute window during the mid-afternoon convective heating maximum. Despite the destructive severe weather, upper air conditions were marginal for supporting organised thunderstorms, highlighting the importance of capturing mesoscale processes, such as the southeasterly change. To further understand possible discriminators of the four highimpact BMA cases, a 10 year climatology of the mesoscale and synoptic environment associated with southeasterly change events was developed for the warm season months of November to January. It is shown that while only a small number of events are associated with high-impact BMA thunderstorms, these events share a set of conditions relating to the pre-change wind shift, timing of the southeasterly change and radar signatures.